1. FIELD OF THE INVENTION
The invention relates to a layered composite construction material, especially materials intended for sliding and friction elements, in which there is applied to a substrate, for example a steel layer, an intermediate layer containing copper and/or tin, and on the latter but separated therefrom by a diffusion barrier layer, there is applied a functional layer, as for example a frictional or sliding layer which contains tin and/or copper. In such materials the diffusion barrier layer can be about 0.5 to 5 um thick, and can consist of a metallic substance which exists in an essentially cubic face-centered crystal lattice.
2. DESCRIPTION OF THE RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR .sctn..sctn.1.97-1.99
In known layered composite construction materials, for example those in accordance with German Offenlegungsschrift (Preliminary Published Patent) No. 2,853,724, there are provided an intermediate layer of a material having emergency running characteristics, for example lead bronze, and a superimposed functional layer, for example a sliding or frictional layer. It is recommended that a thin, oxide-free diffusion barrier layer be arranged between the intermediate layer and the functional sliding or frictional layer, in order to trap tin which migrates out of the sliding layer at elevated temperatures, thereby to avoid the formation of brittle intermetallic phases made of tin and the copper of the lead bronze. The latter, because of its brittle character, would give rise to a loosening of the sliding or frictional layer during dynamic stressing of a sliding or frictional element.
Therefore it is proposed in German Offenlegungsschrift No. 2,853,724 to provide, as the diffusion barrier layer between the intermediate layer and the sliding or frictional layer, a sputtered diffusion barrier layer made of NiCr.sub.20 or pure chromium. As experience in practice has shown, however, the nickel-chromium diffusion barrier layer and also a pure chromium diffusion barrier layer are not completely effective with sliding or frictional layers which contain tin (e.g. those made of AlSn alloys). Investigations with the aid of the scanning electron microscope and the microprobe show that tin evidently diffuses via lattice vacancy sites through such a sputtered diffusion barrier layer made of NiCr.sub.20 or pure tin, and combines with the copper of the lead bronze intermediate layer to give a brittle intermetallic phase, such as Cu.sub.6 Sn.sub.5 which, under conditions similar to practical operation, leads to a loosening of the sliding layer and thus to failure of the bearing.
Galvanically applied diffusion barrier layers also have found in practice to exhibit a comparatively large number of lattice vacancy sites and therefore at elevated temperature they likewise allow tin to diffuse out of the functional, sliding or frictional layer, into the intermediate layer.